The invention relates generally to the field of blood pumps. More specifically, the invention pertains to continuous flow pumps of rotary design, suitable for permanent implantation in humans for use as chronic ventricular assist devices.
Thousands of heart patients who suffer from severe left ventricular heart failure could benefit from cardiac transplantation. However, owning to a shortage of donor hearts, most of these patients face a foreshortened life span characterized by frequent hospitalizations, severe physical disability, and death from congestive failure or cardiogenic shock. If a left ventricular assist device (xe2x80x9cLVADxe2x80x9d) were available for chronic use, many of these patients could be returned to prolonged and productive lives.
Prior art LVADs, now in clinical trials, provide a cyclic or pulsating delivery of blood, designed to emulate the natural pulsatile blood flow through the heart. This design approach has resulted in a variety of anatomic and engineering problems. Cyclic delivery systems tend to be physically large, making implantation difficult or impossible for some patients. Cyclic delivery systems also employ artificial valves, having special material, longevity, and performance requirements. All of these characteristics make cyclic blood pumping device both complex and expensive.
It is apparent that if the requirement of pulsatile blood flow is eliminated, the LVAD could be much smaller, simpler, and less expensive. Rotary pumps, whether of centrifugal or axial flow design, provide substantially continuous liquid flow, and potentially enjoy a number of the listed advantages over cyclic delivery systems. However, the prior art has not developed a durable rotary blood pump, owing to unique problems with the rotary pump""s driveshaft seal. In a blood environment, such driveshaft seals have a short life, and contribute to a premature failure of the pump. Prior art driveshaft seals may also cause embolisms, resulting in a stroke or even death for the patient.
Accordingly, it is an object of the present invention to provide an improved rotary blood pump, by eliminating the necessity for a driveshaft seal;
It is a further object of the present invention to provide a compact, rotary blood pump using passive, magnetic radial bearings to maintain an impeller and its support shaft for rotation about an axis;
It is yet a further object of the present invention to provide a rotary blood pump having bi-stable operation, in which the impeller and the support shaft shuttle as a unit, between two predetermined axial positions;
It is another object of the present invention to provide blood immersed axial thrust bearings which are regularly washed by fresh blood flow to prevent thrombosis from occurring;
It is yet another object of the present invention to provide a unique thick bladed pump impeller, which houses both motor magnets and radial bearing magnets, and includes narrow, deep, blood flow passages;
It is yet another object of the present invention to provide a pump impeller which is effective pumping viscous liquids, such as blood, at low flow rates, and which minimizes hemolysis of the blood by using only a few pump impeller blades.
In accordance with illustrative embodiments of the present invention, a rotary blood pump includes a housing and a pump rotor. A centrifugal pump impeller is attached to an impeller support shaft, or spindle, to form the pump rotor. The pump housing includes an elongated inlet tube surrounding the shaft, and a scroll-shaped casing, or volute, with a discharge outlet, enclosing the impeller.
The shaft and the impeller are specially suspended within the housing. Radial magnetic bearings of passive design, maintain the support shaft and the impeller about a rotational axis. The magnetic bearing which levitates the shaft includes a plurality of permanent ring magnets and pole pieces arranged along surrounding portions of the inlet tube, and a plurality of permanent disc magnets and pole pieces within the shaft itself. Radially adjacent pairs of these magnets are of like polarity. One part of the magnetic bearing, which maintains the impeller about a rotational axis, includes a plurality of permanent rod or arcuate magnets disposed in spaced, circular relation around blade sectors of the impeller; another part of the bearing includes a pair of permanent ring magnets outside the casing, on either side of the impeller. Adjacent portions of the rod and ring magnets are of opposite polarity.
The shaft and impeller are axially restrained by a magnetic and hydrodynamic forces in combination with mechanical thrust bearings, or touchdowns. The magnets of the magnetic bearing in the inlet tube and shaft may be arranged in slightly offset axial relation, to produce a translational loading force, or bias, along the longitudinal axis of the rotor. This bias substantially counteracts the axial force resulting from the hydraulic thrust of the rotating impeller. However, the hydraulic thrust will vary as a function of the cardiac cycle and additional restraints are desirable to ensure that pump operation is stable and controlled. For this purpose, a pair of blood immersed thrust bearings is provided. These thrust bearings may be located at either end of the rotor, although other arrangements are feasible.
One thrust bearing is included at the upstream end of the support shaft, and the other thrust bearing is located on the bottom, or downstream side of the impeller. A spider within the inlet tube includes a touchdown, or thrust surface, against which the end of the shaft periodically touches. Another touchdown is provided on an inner surface of the casing base, adjacent a downstream terminus of the impeller. A predetermined amount of spacing is included between the two touchdowns, so as to allow the shaft/impeller assembly axially to shuttle back and forth, in response to the user""s cardiac cycle. This shuttling motion will produce a pumping action, frequently exchanging blood in the touchdown area with fresh blood from the circulation. This pumping action minimizes the likelihood of blood thrombosis in the thrust region, by maintaining the blood at an acceptable temperature and by shortening its residence time in the thrust bearing gap.
The impeller is of unique configuration and characteristics, owing to the special requirements of the present application. Contrary to conventional centrifugal pump design, the present invention uses relatively few impeller blades, generally resembling pie-shaped sectors. Moreover, the blades are made quite thick in an axial direction, having deep and narrow, arcuate channels between adjacent blades for the passage of blood through the impeller. The substantial height of the blades provides a relatively large blade working surface, ensuring efficient pump operation. These structural features decrease hemolysis of the blood, while maintaining useful efficiency in a pump using so few impeller blades.
Sealed, hollow chambers are provided within the thick impeller blades to reduce the density of the impeller. These chambers reduce gravity induced loads on the thrust bearings, which in turn reduces the likelihood of thrombosis of the blood used to lubricate the bearings.
The thick impeller blades are also used advantageously to house magnets used in the pump drive system. Torque drive is imparted to the impeller by magnetic interaction between arcuate, permanent magnetic segments imbedded within each impeller blade sector, and a circular electromagnetic stator, affixed to the casing. Back-EMF sensing is used to commutate the brushless motor stator, providing attractive and repulsive forces upon the magnetic segments. A control unit and a portable power supply, worn on the user, power the pump drive system. The control unit allows the speed and drive cycle of the motor either to be programmed or interactively determined by the user""s physical activity or condition.
In certain embodiments of the invention, the motor includes a plurality of permanent magnets carried by the impeller and a motor stator including an electrically conductive coil located within the housing. A ring of back iron is fixed to the casing to aid in completing a flux return path for the permanent magnets and to decrease the axial thrust which results from the attraction of the motor rotor magnets toward the motor rotor stator. The impeller has a forward side facing the inlet tube and a rear side downstream of the forward side. In one embodiment, the conductive coil of the motor stator is located adjacent the rear side of the impeller, and a stator back iron ring is located outside of the conductive coil, within the housing and fixed to the housing. In one embodiment, a second ring of back iron is located on the forward side of the impeller and outside of the casing but inside of the housing, with the second ring of back iron being fixed to the casing. In that embodiment, a second motor stator having an electrically conductive coil is located on the forward side of the impeller outside of the casing but inside of the housing. In that embodiment, the second ring of back iron is located forward of the second motor stator.
In certain embodiments, a plurality of hydrodynamic thrust bearings are located outside of the axis of rotation of the rotor. The hydrodynamic bearings are wedge-shaped and, during rotation of the rotor and impeller, the hydrodynamic bearings are separated from the casing by a fluid film and are not in direct mechanical contact with the casing.
A more detailed explanation of the invention is provided in the following description and claims, and is illustrated in the accompanying drawings.